I do not really think any built-in image editors are up to the sophistication of something like Photoshop in terms of creating images from scratch, so the best alternative is to create the image in Photoshop/similar and convert the image to thread colours in the thread conversion program.
Now, you have mentioned two versions of the same program, PCStitch & MacStitch. The conversions produced by these programs are not as good as 'Pattern Maker'. So I would recommend of the two, consider Pattern Maker (PM). PM is the software used by Heaven & Earth Designs (HaED). The conversions by PC/MacStitch are frequently quite poor & you have very little control on the result. I know a lot of people rave about the quality of PCStitch's conversions, but the quality is really quite disappointing if, like me, you are very picky. PM is generally quite good, but looses definition in dark areas, resorting to large monochrome blobs. This, unfortunately, is visible in some of the HaED charts and is, frankly, not good enough.
However, I would like to suggest a 'New Kid on the Block', a free to download & use Windows program called 'BlendThreads' if you are after the ultimate in accuracy and faithfulness to the initial image. I admit to an interest - I'm the programmer & wanted extreme faithfulness to the image. Pattern Maker and PC/Mac Stitch have a limit of 240 colours in their conversion, and blended colours have to be calculated by the user and manually entered as new thread colours. Blend Threads has no limits on the numbers of colours and at the tick of a box, blended colours can be used - adding 12000 extra possible colours. I have done (but not stitched!) a conversion that with all settings set to extreme accuracy, gave a 7000 colour conversion. That is hard to beat for accuracy, and probably near impossible to stitch. The program allows the user to decide their own limits, rather than have them dictated by the program.
Take a look at a number of images converted using BlendThreads:
1) 'The Ionian Mission' by SteveM. This includes a comparison between Pattern Maker and BlendThreads and shows how Steve was going to the nth degree for precision & accuracy & in the end used BlendThreads to create his 350 colour chart. This can be seen here: http://www.crossstitchforum.com/viewtopic.php?f=6&t=28279
2) 'Honeymoon Portrait'. This is an early conversion using the program before I added extra dithering options & the ability to blend threads. I kept the colour count down to 118 (thread here: http://www.crossstitchforum.com/viewtopic.php?f=6&t=26356
3) 'I'm watching You'. First blended thread test piece with the program. 117 colours from 156 floss colours (thread here: http://www.crossstitchforum.com/viewtopic.php?f=6&t=27900
Actual stitched result:
4) 'Gryff'. My current unfinished photo conversion of my cat in the snow (thread in the in the 'Stitch-a-long' forum http://www.crossstitchforum.com/viewtopic.php?f=6&t=28246
5) 'Luz'. My current conversion of a Luis Royo painting (with permission from the artist). Thread in the SAL forum - http://www.crossstitchforum.com/viewtopic.php?f=6&t=28245
6) 'Color of the Wolf'. Allyn is re-doing a previous image using BlendThreads, after she couldn't continue with the original conversion (it was just too dreadful). Allyn hasn't got too far yet, and is what has been completed is in this thread: http://www.crossstitchforum.com/viewtopic.php?f=6&t=28274
. The abandoned original is here: http://www.crossstitchforum.com/viewtopic.php?f=6&t=27802
At the moment Blend Threads uses the same public domain colour component values as PM and PC/MacStitch for the DMC range of threads, so any colour errors relating to the colour definition files will be shared between all three programs. It is known that the colour definition files are not completely correct (on average they are too light by around 10%), and I am trying to get a chance to photograph a DMC floss card in good lighting conditions so I can create a more faithful thread definition file. This is very weather dependant and so far I have been unable to get the light I want before doing it. The Anchor thread colour definitions, and the other ranges supplied (including Lego Bricks, 2010 colour range), are all original data for BlendThreads and do not exactly match any other program's thread colour definition data.
I hope this is of interest, and as BlendThreads is a freebie (by way of a thank-you to forum members for their help), it may be worth trying anyway. The program web page is on my website, here: http://www.chestnutpens.co.uk/misc/crossstitchplan.html
and can be downloaded from that page or my signature.