When you click "New Topic", you'll see, at the bottom left side, "Additional Options". Click on that, and you'll be given the option to attach a file. If you want the forum thumbnail picture to show properly, attach the image file first, then click on "More attachments" and attach the file with your sim.
Note, you can only upload one image as an attachment. Any other images you want to share must be put in the body of your post and be hosted on the third party image host of your choice (Photobucket, Imageshack, etc). Please give full credit for any custom content you are including with your sim, including the name of the creator and and a link to the site it's from. A Clean Installer screen shot will work for this purpose if you can't remember the name of everyone and where you got it from. Also, please make sure you're not violating anyone TOU by including it with your sim, or using anything from TSR. Link to those items instead and, in the case of pay items, you may also link to the booty or any other site offering for free as long as you also include links to the original pay site it comes from.
Please don't hesitate to ask if there is anything else you're not sure of.
I look forward to seeing your sim!
