Looks like Gmail added another new feature!

You now have the option of viewing or downloading image attachments! Read on for details…

When you receive an image file as an attachment (like .jpg, .gif, or .bmp) Gmail now displays two links for each attachment: One is the filename, and the other says “Download”. Clicking on the name link opens a new browser window and displays the image. Clicking the Download link initiates a standard file download.

Other attachment types simply display the filename letting you download the file. Nice touch!