If it is just a picture from somewhere else on the net you can hotlink (although some sites frown on this and will swap out your image with a message chastising the hotlinker but most places don't care anymore and this is such an obscure forum it won't really dent their bandwidth limits... even then they might be having the image hosted somewhere else anyway so it becomes moot).
To hotlink an image and post to the Cake forums:
1) Find a silly image.
2) Right click the image and select Copy Link Location (this will place the images URL onto the clipboard).
3) Navigate to the thread you want to post it in then select the Insert/Edit Image button at the far left of the second row of editing tools above the comment box.
4) A dialog will pop up. Right click and paste (or Ctrl V) the URL into the box and click Insert.
5) Click preview to make sure the image is going to load then Submit.
6) Sit back and be showered with the praise and awe of your peers.
If you want to use an image on your computer you have to upload it somewhere like an image host as Craig mentioned. You have to create an account with that host. Different hosts work different but it's usually just a variation of clicking "Upload" then selecting an image from somewhere on your computer (it will open folders on your computer and you navigate to where the picture is and select it). Once it is uploaded to the host there will usually be a list of code options like IMG, BB, HTML or plain URL codes. I forget which codes the new forum accepts but it's probably HTML or BB. You just copy that and toss it into the comment box or you can copy the raw URL and use the Insert Image button as described above.
A couple of decent free hosts are Imageshack and tumblr. Photobucket used to be good but now it's a pain to use and a flashscript nightmare. It's basically become the Myspace of image hosts.