Whether you have Paintbrush depends on what version of Windows you have. It does not generally come with WinXP, in versions I've seen; although I believe there are/were at least seven different WinXP versions available. I don't have it on my WinXP, but "she who drives a Win2K" does. The "Microsoft Picture Viewer" that's the normal default "looker" with WinXP doesn't allow you to change the filetype of a picture.
If your friend took pictures and got them into .bmp format, it's quite likely friend has some sort of image editing program. Your simplest solution might be to ask friend to send them again in .jpg format. Friends sometimes will do that sort of thing.
You might also have another friend you could send the .bmp files to (email?) who could convert them for you and send them back.
With either of the above options, be sure to tell them what eBay says you need, so that you only have to do it once.
You can use one of the programs suggested, and conversions of this kind are pretty simple with almost any of the available programs; but you will have to learn at least a few buttons to click to be able to do it yourself. It can be a lot of fun, but you may not want to get into it just for this one minor fix.
The basic answer is that it's a simple thing to do, if you have, or a friend has, a program that's able to do it.