Via the Shrink Me FAQ:

• “Why should I compress images? Image compression is ideal if you want to make your site / app load faster or to save some storage.
• Do compressed images suffer from quality loss? No, your images should look identical to the human eye. You almost won’t see any differences, just give it a try.
• What Compression Ratio is used for JPG images? Shrink Me’s compression ratio for JPG images is 60%.  In the future, you will be able to change this value.
• What is the maximum file size? Currently there is no maximum file size limit. Beware that big file sizes will take longer to compress.
• Can I compress multiple files at once? Yes, you can shrink as many as you like at once.
• Why didn’t my image get any smaller? This may happen to images which already have been optimized.”