I find using photobucket the easiest way to put images on another website . Look for a picture you want on there . There will be a list of codes to the left of the image , look for the one that says IMG code . Click it and it will be automatically copied . Paste the code in , post your comment and the image will show up :)
Right click on the photo and choose either "copy image", or "save image as" (save it to a place on your computer where you'll remember it). If the form allows you to, you can then right click in the comment box and select "paste", or search for an option to upload a photo - find the photo you saved and upload it.