The computer you are using has nothing to do with the size of the image or the message you received. That message is produced by the software on SCI, which limits the size of image or file that you can upload from your computer, when you try to use the "attach" function in the forum. You must resize the photo to attach photos to you posts. However, SCI also has a photo hosting gallery which will take any size image; if you upload your photos to that gallery, you can then post images within your posts by using the icon in the "go advanced" post option. You can also use any other image gallery on line, such as Photobucket.
You can use export photo in iPhoto and specify a size in terms of pixels and resolution. Send them to your desktop and then check the file size to make sure it's not too big, and if you have the size right in terms of pixels then just lower the resolution a bit.
I like the free Photobucket service. I post my photos there, size them to import to SCI and the other photos are there if anyone wants to check out model builds, scenery photos or track pix. It it ain't a MAC, I don't use it.