As labeelady says, your photo is too large a format - you can see how big it is if you go into 'properties' - also good to find this so you can check whether or not you've successfully reduced it's size. You should have an 'edit' option somewhere on the toolbar when your photo is open. If you do, shrink it to 75% or thereabouts. Remember to save the new reduced image. Once saved, you just upload from message toolbar.
If you don't have an edit option on the page you are viewing your photo on you!ll have to try another application. Open the photo you want to use as normal. Right click somewhere on the photo and a drop-down menu should appear with a selection of options. Choose the 'Open with' and a list of viewers will appear. Choose one of them. Do you have Microsoft Office or similar? If you do, that is easy to use. There are probably other viewers too.
the main thing is to reduce the size of the photo and save it. There's info under the board Help section too. Good luck!