I'm aware this was asked some months ago but this recently became a part of my weekly workflow and I decided there had to be a better answer than the roundabout answers that were given.
My solution is admittedly a little hacky but it has worked for me and, barring any future changes Google plans to make in how they encode their URLs, should work for the ...
I recently had to do this and was able to use search/replace tools.
Select the cells with image URLS.
Edit -> Find and replace (ctrl+H)
Search expression: (.*)
Replace expression: =IMAGE("$1")
Match entire cell contents
Search using regular expressions