Welcome to WikiTree.
Firstly, the image needs to be already uploaded to WikiTree.
When you go to give an answer at the Photo challenge, up at the top of the text section of the answer box there are a heap of icons. You should be able to see an elongated circle next to an A icon that looks like the link in a chain. That is for adding a link into the text section by highlighting a section of text and then clicking the icon, adding your link to the link area and clicking ok. Two icons over to the right there is a square with what looks like two hills. That Icon is for adding an image into the answer box. Search the Image and add it. Add your text that you wish to add with the Image. Then before you hit ok, just check the size of the image by scrolling down and check that the width box is set at approx 500, but definitely less than 1000.
I hope that helps.