Saving single Photoshop Brushes
Open the image that you want to create the Photoshop brush from, then press ctrl + A on the keyboard, or go to the select menu (at the top) and click on 'all'. You should now have a selection running round the outside of the image.
If you just want to select part of the image as a brush, you can use the marquee tool simply click and drag until you have selected the section of the image you require. note the feather on the marquee tool needs to be set at '0' or it won't work.
Now the image is selected go into the brushes menu (see figure 1 on the left), and select 'define brush' from the sub menu.
This will add the new photoshop brush to the brushes menu. Photoshop creates a small thumbnail for each new brush. On large brushes it creates the thumbnail from the top left corner of the image, if there's nothing in the top left corner then the thumbnail can appear blank. I have not yet found a solution to this problem.
Here's the new photoshop brush in the brushes menu. To save a single brush you need to delete all the other brushes in the menu, to do this right click on each brush one by one and in the new menu select 'delete brush'. in Photoshop 5.5 you can click on a brush while holding down the Ctrl key (scissors icon appears) this is a very quick way of deleting brushes.
You should be left with just one Photoshop brush, in the sub menu go to 'save brushes' and then name the brush accordingly.
Saving single brushes is handy for creating new brush sets, e.g. if you download a set and you only like one brush from it, then you can save that single brush and add it to a new brush set. See Saving Photoshop Brushes for more details.