Here is an easy way to do it if you just want to add a Happy Birthday picture for example:
Use Google to find a birthday image. Go to www.google.co.uk and search for Happy Birthday. Click on images (top left):
googlesearch.jpg
Then click on search images:
googlesearch1.png
Now browse for the picture you want to use and click on it to open it up full screen.
Press the right mouse button and select "Copy" from the pull down list.
Now create the post you want to and where you want to insert the picture you simply press "Ctrl v" to paste the picture in.