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
"I was sending it as an attachment"
The provided answer is true when adding/dropping an image directly into the body of the email but has nothing to do with attachments.
We electronically sent images to people all the time. We never had an issue like that before when sending as an attachment. There must had been something else there.
In any case -as an ...