Lily Allen. Image from parentdish.co.uk
Millie Mackintosh. Image from nowmagazine.co.uk
It appears that Lily is talking about the recent trend of bikini and gym selfies in which people take pictures of their bodies, honed by the gym for all to see. Whilst she could be referring to any number of celebrities that do this such as TOWIE's Lucy Mecklenburgh or MIC's Lucy Watson. It cannot be a coincidence that Millie Mackintosh posted a bikini selfie, showing off the results of her gym workout around 20 hours before Lily posted this tweet. Millie posted this particular Instagram photo with the caption 'Hard work in the gym finally paying off! #nopainnogain', which she also posted to her Twitter page.
Lily's tweet mentions people who post pictures of themselves 'working out' and Millie's picture makes a reference to the 'gym' and the fact that she got that body through working out. Furthermore Millie has posted numerous other pictures of herself working out with her personal trainer before, so it seems likely that Lily is making a dig at her in particular.
Thin people who incessantly feel the need to post pictures of themselves 'working out' you are smug, not inspirational. Stop it. #living ugh
Lucy Mecklenburgh working out. Image from uk.omg.yahoo.com
Lucy Watson body selfie. Image from ok.co.uk