Hosting on your own site is the best way, but you will have to be prepared to re-size the image to meet PPRuNe requirements of 800 x 600.
Alternatively why not create a PPRuNe Album? That's what it has been set for, go to your User CP, Networks on the left side has 'Pictures & Albums', click and follow the bouncing ball. The peculiarity of the way it has been set up by IB requires the image to be approved by a moderator but once done it will be there in a decent size instead of the attachment process that you are using.