JessicaMarie, the easiest way to add a pic to a spoiler is to upload the picture as normal into the regular part of the post, but before submitting, highlight the picture and cut (Ctrl + x). Open up a spoiler window, then paste the picture (Ctrl + v) into the spoilers box. Add text details below (it's hard to tell box contents from the pic, we all appreciate when people add a list below the pic of what they got), then click OK on the spoiler window and submit the post!
And Zadidoll taught me how to do this. She's amazing!