Sorry for not responding earlier -- I saw the post in my subscriptions and meant to type something up but got distracted and forgot to do so!
If you have a loose image and are at a table full of other loose passives that will call you, 77/88 do not hold up well against 3-4 others in the same pot, especially out of position. It's better to do as you are doing -- limp/complete it and hope to flop a set or with a suited connector to flop nuts or a good draw.
With a loose image at a table full of limping TAGs, raising might get a few of them to fold but if your image is super loose then you might just get a lot of callers (which is bad in this case). If you have a TAG image, a raise will probably get a lot of the limpers to fold so you can isolate and then more easily take down the pot postflop with a cbet, and it is much easier to know where you are in the hand when isolating down to one other player.
Just posting on here again trying to get more responses on this topic. I still feel kinda lost with hands like 7s and 8s. Iv been completing with a lot of limpers and raising against a single limper. Mainly because i feel it's really hard to be ahead in multiway pots with these hands, and I end up just set mining basically.
I think this is generally the best way to play medium/small pocket pairs in this spot. I think the reason for a raise in this spot isn't so much a value bet because you think you are ahead, but rather to take the dead money offered by the limper and to get information if they decide to make a call after the raise.
Again though, if the single limper is a loose calling station that does cbet on occasion then it may be better actually to just complete rather than raise, because it's rare to hit a flop with mid/low pocket pairs that we can feel really confident about (plenty of overs to worry about). If we do hit our set or monster, even though we limped preflop the loose calling station might call us down light anyways which will build the pot when we bet each street.