Images on the Internet have direct URLs. And to add an image to your Polydojo form, you'll need the image's URL. Consider the following image/meme:
The meme's (i.e. image's) URL is:
https://i.imgflip.com/265n7x.jpg. Try clicking on it!
On Chrome, right-click on the image and choose 'Copy image address'. On Firefox, right-click and select 'Choose Image Location'. Other browsers too have a similar right-click based options.
Polydojo engineers are currently working on introducing a computer-uploaded images.
In the mean time, you may use a 3rd party service to host your image and get it's URL. Free image hosting services include Imgur, ImgBB, TinyPic etc. Please read their terms before uploading images.