Design web graphics with Pages more easily

I need to assemble a couple of individual pictures together, plus some nice text, and the result has to be a PNG of a certain pixel size.

I'm not a web-designer, so, I don't have any specialised software, and since I need to do this like once or twice a year, it probably won't be worthwhile to buy and learn how to use Photoshop or some such.

I've been (mis)using Apple Pages for these kinds of tasks, which generally works great, except for this pixel requirement for this specific task of a web-graphic.

What I found that works is:

• design the document in Pages, with cm as the measurement system, and using a calculator to ensure proportions
• use the Fit Width zoom option, and resize the window next to a pixel screen ruler
• use ⇧⌘4 to take a screenshot of a portion of the screen (usually of the whole window, e.g. with a space and a button click)
• use Preview to cut an exact PNG of needed size from the above screenshot
• if size is off, repeat the last three steps

Is there a way to reduce the above steps, and make the process somewhat less ad-hoc? I've originally started with exporting from Pages into PDF, and then doing the resize magic with PDF, where I still had to take the screenshot of it before going further, but then realised it was an extra step that was not necessary.