I would be terribly upset if my phone's pictures were accidentally deleted, as these are treasured memories with friends and family, and are so very important. If this occurred while downloading an application, I'd really like to know what application it was, to see if this is an issue. Did you get a confirmation message about losing your SD card data? Did you get a message that your memory space was low, and would you like to override and download anyway? This could be why the pictures are no longer there, but I would definitely remove the application to be on the safe side. (Menu>Settings>Applications>Manage Applications>All. Then, find the application, click on it, then click "Uninstall.")
On another note, bethobrn, I have good news for you!
Yes, you have great possibility to retrieve the pictures deleted or formatted from cellphone. Those pics are still in the memory of your phone just not visible, and pics data are waiting to be written over by new data. **Important** From now on don't use your cellphone to take more pictures. This can cause new information to be stored, and remove or corrupt the data that is still there.
To retrieve your deleted pictures is to use some picture recovery programs. One is Asoftech Photo Recovery, but there are more you can use. Here is the steps you may refer to with Asoftech to retrieve your deleted pictures from your phone memory.