Just for clarity make sure you are using the "Gmail" app and not just reading Gmail through the "Email" app.
When I receive jpeg attachments in Gmail on my Droid and I scroll down, I will usually see the picture displayed inline after the text of the email. At the very bottom is allso a "Download" button and a "Preview" button.
I don't really know if there's a size limit on attachments when it comes to viewing on the Android Gmail client.
I have done some testing and the problem seems to be with pictures in line..
I tried attaching jpegs and I could seem them..
I did a second email that I inserted the picture in the email itself and I could not see them.. This does not make sense to me because I get pictures in other emails like advertisements and I would think those would be included in line.
It is pictures inside the body. There is another topic on this around here and many many out there on the Android issues forums (including the main Android OS bug forum). Dates back to the G1 so hopefully Google will correct this issue. Again...only when someone does something like "insert an picture" using outlook, where it actually puts it in the body. All other types of attachments seem to work fine.