When gaming on a Virtual Table Top (VTT) the one thing I never seem to have enough of is tokens. So I like to scour sites like Deviant Art and make my own with TokenTool from RPTools.net.
You simply choose a size, border, drag an image from your computer onto it and reposition as needed. The results can then be saved as a PNG. This saves a ton of time compared to creating each token in Photoshop or GIMP.
To reduce the size of the resulting PNG from TokenTool, you can run it through something like PNGGauntlet. Simply drag the PNG onto PNGGauntlet, select "Overwrite Original Files" and click Optimize.
This will cut down on download times for your players. Please note that running PNGGauntlet or any similar tool will change the hash of the file. So if you are currently using an asset on a VTT like MapTool, this may break your campaign file.
Sprite sheets combine image assets into one giant image. This should speed up downloading (compared to download each individual image) and drawing to screen. That said, I don't know of any VTTs that support sprite sheets currently.
If you want to try creating sprite sheets there are several utilities available. One of the simpler options (but not free) is Texture Packer.