If adding files that were imported to your computer directly from your digital camera there is a good chance they are very large files. Re-sizing them before attempting to upload them to BPL (or other Web sites as well) is recommended. A suggested size would be in the 800x600 pixel range as this will provide an optimal viewing size for most people's computer screens.
Since you mentioned you were using Apple's iPhoto an easy way to do what I suggested above is to click on the thumbnail of the photo in iPhoto, and select "Export" from the "File" menu. From the dialog box that opens you can tell iPhoto to export a .jpg file of medium quality and medium size to be saved whereever you choose (say your Desktop for ease of finding it). Once saved you can then use the "Insert Image at Cursor" button on the BPL forum, click "Browse", find the file you just saved, upload and all should be well.
* note, I should add that I posted this long description for others who may be encountering this same issue, as you have obviously figured it out judging by the lovely tarp photo you posted ; )