Click on 'Post Reply' then select the little 'Tree' above where you are typing.
Click 'Choose File' ; then 'Upload'.
Files have to be of a smaller size than you would normally store them so you may have to resize.
Re-sizing is easy to do and it will save a smaller copy.
Go into EDIT then click on 'Properties'. From the drop down menu select 'Resize' then alter the file size. I find selecting 600x seems to reduce the file size nicely.
That will then be saved as a copy which you select to upload.
Best to do the re-sizing before you go into 'Post Reply' so that it's ready to upload.
Have fun.
Al.