Actually Google checks the files pointed by <img src="..."> tags, rather to look up for jpg items. Images can be in any format, there is no restriction of using jpegs only while you have png, gif, wbmp and so on.
There are two things you should do:
1. Put alt attribute to all images, with a descriptive content
2. Enable the advanced image search in the Google Webmaster tools for this web site, if you use this Google Service.
I hope this helps.
the Telerik team