Firefox and Opera have an image information dialog (right click on the image) which displays the format of the image, however not all versions detect the correct format. To check whether it works in your browser, look at the image information of this image. If it says PNG, it's detected the format correctly. If it says JPEG, it hasn't.
Various (command line) ways to find the format of an image:
identify FILENAME(requires imagemagick)
sips -g all FILENAME(OSX only?)
- [last resort]
hexdump -C FILENAME | head -n 1(most image formats have headers that describe their type, notably excluding JPEG, but JPEG files begin with 'ff d8')
Getting better images from various sites:
170x170 (or sometimes
100x100) in the image URL with
1200x1200. Be wary of upscaled images if using the 1200x1200 versions.
._SS400_.jpg at the end of the URL with just
.jpg (sometimes the letters and numbers are slightly different but it always starts with a dot and an underscore). This will usually get rid of any white borders around the image too.
Fore Firefox users, https://addons.mozilla.org/fr/firefox/addon/picturefox/ might be useful.
Click the image to go to the page with the full images. Even when there isn't a "more images" link, the page will often have a larger front image.
1.200.jpg, etc., at the end of the URL with
cover_160.jpg, etc., at the end of the URL with
serve/500, etc., with
t500x500.jpg (or whatever) with
original.jpg at the end of the URL (you can just remove the question mark after
.jpg and anything after it)
You need to register/log in to access high resolution images.