First, take a look at how big your files actually are. Right-click on a file's icon and select 'Properties'. The ideal size for an eBay image is around 30Kb to 80Kb (and certainly not much bigger). A typical image taken on a digital camera will have a file size of between 800Kb and 3000Kb, so you probably need to reduce the file size considerably.
You've probably got suitable software available already, as it almost certainly came with your camera. All you need to do is to view the image in that software and look for a 'resize' option.
If that's a bit vague (or you've not got any suitable software), download and install Irfanview. (It's a free image viewing and editing program, which deserves a place on everyone's PC):
http://www.irfanview.com/
Open your picture in Irfanview.
Go to Image > Resize/Resample.
Click to put a tick alongside '640 x 480' (That's probably a suitable size)
Click 'OK'
Then go to File > Save As
Choose a new name for your image (e.g. 'vasesmall' instead of 'vase') and ensure that the format is selected as 'JPG' (not 'BMP', which will probably show as the default).
Click 'Save'
You'll then have an image which eBay will accept.
Chris