I have sent photos via iPhoto in the past. Had to reset my email address

When I attempt to email a photo using iPhoto, a yellow triangle pops up with an exclamation point. No text, no instructions, and only option is to click OK. IWhat do I do? I have sent photos via iPhoto in the past, no problem. Had to reset my email address in preferences for some reason , and a request for iPhoto to share my photos or something came up and I just clicked OK thinking I needed to do that to email them. Now, yellow triangle. ( I have changed my preferenced to send via my email program, but I prefer iPhotos snappy layouts and would like to correct this problem.)