that's a good question 'wife99. i can only tell you how i see it.
let's say a person starts of talking about a quilt they saw that had bras appliqued to it. then somebody jokes that her own bra would be so small she'd have to use it on a doll quilt. or somebody jokes that hers would have to go onto a king size. the next thing you know, we're all joking around about bras and what goes in them. most of us are laughing 'til we cry as we go through the different comments, BUT we have - in fact - hijacked the thread. not everbody would appreciate that.
the other example you presented would also be a hijack because the first member posted to show off her work and get some feedback to either validate her success. if her post inspires you to show us your similar work, you should start your own thread to show yours. it's especially polite to acknowledge the first poster's as having given you the courage to show yours, or something similar.
there are situations in which posting your own pics in somebody else's thread would not be a hijack. let's say i swallowed my pride and posted pics of one of my disasters. then suppose i asked "where did i go wrong?" or "how do i fix it?" if somebody else posts a photo to illustrate their suggestion and make it easier to understand, that is not a hijack. it's a contribution to the conversation.
all you have to do before posting in somebody else's topic is ask yourself how you would feel if it was yours and another member added what you're thinking of tossing in. 99.975% of the time, you'll know what's right. if you make a mistake we'll just tar you, feather you, run you out of town on a rail, and maybe throw some darts at your picture. nothing drastic. or forgive you and move on, whichever involves the least work. :lol:
P.S. theee funniest thing just hit me. this thread is about getting suggestions for reworking the General ChitChat section. so, by going off on a tangent about hijacking, we have just hijacked the thread. :lol: :lol: :lol: