You can use Motion to separate your yourself from the background. I don't know how you're doing it now, but you could do it a couple of ways.
-Shoot green/blue screen and use the keyer to separate yourself (easiest).
-Use the masking tools to cut yourself out.
As for doing effects, I don't know the site you are talking about, but Motion comes with like, more than a hundred filters. You can use these mixed with masks for anything (effects, color correction). Not to sound cliché, but the only limits are based on your skill level and imagination.
your workflow is - sorry to say that - a bit ... inconvenient
the usual workflow is: record using some plain green or blue background, create matte/key new background in, color-process video.
'matting by hand' skips 50 years of video-processing ...
you can accomplish that with iMovie, Motion or FCPX. iM and FCPX come with a bundle of 'presets' which allow similar coloring/contrasting/de-saturation as pixlr. FCPX for sure allows much more, in detail manual manipulations of color, although the 'video' tab in iMovie11 offers enough 'knobs' to create the few looks pixlr offers. Motion wouldn't be my first choice, as the app's name implies, it is basicly not meant for such simple color-corrections.
I only have iMovie06 HD, .. .
when your Mac is that old, it came with 'antique' iMHD6 only, double-check, if it full-fills the min. specs of M5, esp. the graphic-card!
e.g. I invested into 8GB Ram, to make the new stuff (FCPX, M5, Lion) 'useful' on my old hardware (MacMini) ...-