The obligatory link: "Image Spam" could bring the internet to a standstill
Well, the title is certainly overhyped. Beyond that, though, it offers a chance to hand out another tip or two to Outlook users as to how to sort out the junk.
The easy way about it is to go to Tools, Rules and Alerts, New Rule..., Start from a blank rule (radio button), Check messages when they arrive, Next, Step 1: Select condition(s), check the box "which has an attachment", Next, Step 1: Select action(s), check the box "move it to the specified folder", Step 2: Edit the rule description, click on the underlined word specified, select Junk E-mail from that list, Finish.
Now, if your friends send you mail with attachments, you can get around this by creating a new rule for each of their e-mail addresses that automatically puts a copy of any mail from them into the inbox (or any other specified folder), regardless of whether or not it would be sorted into Junk by the previous rule. To do this:
Tools, Rules and Alerts, New Rule..., Start creating a rule from a template, Step 1: Select a template, Move messages from someone to a folder, Step 2: Edit the rule description, click the underlined phrase "people or distribution list", input an address, OK, click the underlined word "specified", select Inbox (or the folder of your choice), OK, Next, Finish. Repeat as necessary until you've added all the people you so desire.
Alternatively, if your friends only send you large images, compared to the images in spam which, from my search through ~1000 in my Junk box, tend to be between 3k and 30k, you can add the condition "with a size in a specific range" to the first rule you created to filter mails with attachments to only filter out mails with small attachments. (The text generally will not account for a great deal of file size.) This is a more questionable tack to take, however, and I'd suggest just leaving it at the first two steps. Really, it depends on how inclined you are to play with the settings to get the results you desire.