Here is another site that will get you sucked in for at least a couple of hours! This site is called "project wonderful" it's actually a very good site for advertising on the net for a fairly reasonable price in most cases.. I would recommend that you have avatars, logo's already to go so that you don't have to loose precious time in setting them up the most popular sizes are 117 x 30, 125 x 125, 234 x 60 and 468 x 60. You can also set up custom campaigns https://www.projectwonderful.com/newcampaign.php so that the site will search for you instead of searching for those sites. This is a wonderful tool to use.
These can be done in programs like photoshop, picassa etc. Then you will need to go in and set up an account (if you haven't already done so) and then go searching for pages that currently don't have any 'bids' on them this is the best way to get yourself into the feel of what you are looking for with your advertising.
Make sure you go and have a look at the website first before you say oh yes that ones free! It might be on a site that you really don't want to have your name associated with. With the ones that are free though you can set up searches that will email you when new spots become available for you to purchase.
This as I mentioned is a fabulous way to kill a morning! and if your blog is doing well with visitor counts then why not add project wonderful to your site and great exposure for doing the blogging that you love to do. The sites can be searched by the number of visitors etc.
Have fun with this. Jenni