There is more than one way to upload your images for sale, some methods are more time consuming than others...
For the Microstock Beginner
Beginners normally opt to upload using their web browser (so called http upload) this is the simplest method and one which almost all internet users are familiar. Most of the decent microstock sites allow you to upload batches of 5 or 10 images at one time making the process a little easier with a 'java' or 'flash' uploader.
The next step in streamlining the upload process is to embed IPTC  metadata  (keywords, description and title) into your images, that way if you upload to more than one site there is no need to re-enter a description on each site. Embedding can either be done with standalone software, an image editor like Photoshop, or one of the microstock submission tools listed below. Further reading on this in our guide IPTC keyword setting software .
The disadvantage of uploading to sites individually is that it is quite time consuming, and each site takes a different approach to batch upload. Submitting to more than a few sites this way becomes quite tedious.
Batch Uploads with FTP 
To assist microstock photographers, the next step up from using the browser upload is to upload in batches via FTP. To do this you will need an FTP client (many available for free) more details in our round-up of the best FTP clients for microstock photographers . Also refer to our quick reference of microstock sites and their FTP address .
It's also important to note that FTP upload is normally used in combination with embedding IPTC keywords  (on some sites this is a requirement). All the leading microstock sites support FTP except for istockphoto.
For many microstock contributors FTP is the best way to streamline their workflow, especially when they submit slightly different batches of images to each site. Some FTP clients will automate the uploading of the same batch of images to multiple sites, and some will allow you to create a queue of preselected images to upload to a chosen site or selection of sites.
Microstock Assistant Software
There are a few pieces of software available aimed specifically at microstock photographers. Some of these tools aid in finding and setting of keywords (embedding IPTC Metadata) as well as uploading images to microstock sites in batches. Some also monitor acceptance and sales of images.
- PicNiche Toolbar 
Firefox Browser plugin supports 6 agencies, with tracking and keywording. Free
- StockPhotoExpress 
I tested back in 2008, but found it somewhat awkward to use(?) some users rate it highly. Free trial. $36.95
- CushyStock 
Process, upload and keyword to 10 agencies, free trial, standard edition $49.95, pro $89.95.
Syndication / Redistribution
For the serious microstocker there are currently a couple of services allowing you to redistribute your images via FTP transfer directly from one server to another. This removes you need to upload images from your computer to each and every site you want to sell those images on, a single upload then distribution to every site you use in a matter of seconds.
Picworkflow (review)  allows you to upload an image once and distribute it to multiple sites, also providing additional keywording and statistics tools.
Photoshelter  provides image storage and website facilitates to photographers, they also bundle in a flexible ftp delivery service which allows you to distribute single images, folders, lists or galleries of images to anyone supporting FTP or secure FTP uploads (see above for details on that). At time of writing photoshelter costs $29.99 monthly for their 'standard service' which allows you to FTP upload and redistribute images. Cheaper accounts have no FTP upload, but there is a free/$1 account option with a 50 MB/month limit you might like to 'try out' with just a few photos.
lookstat  offer professional photographers (and serious amateurs in a position to pay) with a pay-as-you go processing, keywording and uploading service. You need to contact them directly for full pricing details, the service is aimed at photographers whose time is better spent shooting images than keywording and dealing with submission.
Another alternative is to outsource the process to someone in another country who can work for less (ethics aside) I've heard various stories about the low quality of the work that gets done, and difficulty finding someone suitable for the job, and of course you still have to organise a way to get your raw/original files to them to process.
You could also consider employing an local individual or team to do the keywording and uploading etc, so you can exclusively shoot photos and manage other aspects of business. This option allows you to keep a closer eye on the quality of the submissions. This is the way that some of the big names in microstock operate; there is however a fine balance between the increased wages and fees that you will have to pay and small profit margins in microstock.
Read more in Introduction to Microstock Outsourcing