Hey, Megan, I'm here with your requested critique!
Since you specifically pointed out image quality as your biggest problem, I'll start off with that in every section and then switch to the other aspects!
Icons: Image quality is probably the biggest issue in this section, followed by the presentation. As for the image quality, you could try downloading several image pack from our lovely artists, trust me, when you've got lots of lovely pictures, inspiration strikes on it's own!

I know it's sometimes difficult to find pictures in good quality, but the results are so much more than worthy.
As for the presentation, you've got some lovely cuts in here (
+this and
+this in particular), but you should try making sets of icons (more than two icons in generally similliar style) as it would make the whole section look less messy.
Banners: I think you've got an eye for these, and the image quality is not that much of an issue. For example in
+this, the image quality is actually very good, but what's bugging me about it is colouring. It most definitely looks the best in b&w, but if you played around a bit with the first version, I think you could make it very eye-catching. First of all, when you place two people on the banner, make sure that their skin tones match (right now, the girl is more yellow than the guy). Next, you should watch out for the backgound, as it's slightly overpowering. Your background should always have slightly lover contrast than your people, keep that in mind.
Don't cut the chins off. In your second banner (the second after the set, which I'm counting as one banner), the chin is completely cut off, and in the third it comes dangerously close. Chins/big parts of forehead should never be cut unless you're cutting off nose and cheeks as well.

The blending in the first one is very nice. In fact, there are no actual issues with your blending, as it appears neat enough.

Since I'm no expert with Photoplus, I can't give you much advice, but there should be some ways to fade things out. Right now, in some banners, things look rather stacked on top of each other (the clock in the second banner comes to mind in particular...)
Sigs: +this is most definitely my favourite thing in your entire gallery. This is an excellent image quality and you should strive to achieve that in every graphic!

It's just lovely!
Now, in the others, the thing that comes to my mind is text. First of all, you should always have some variety with text... right now you've got same or very similliar fonts and placement. Next, you should always use a dropper tool when picking font colour, to find a colour that's already within the graphic, but not an overpowering one.

As for the blending, not much issue with that, as most of your sigs are one image thing (which is nice, but again, some variety is always good), but in your second (which is your weakest, you might want to remove it

), you should watch out for making people transparent, as this causes graphic to look messy...

Another thing about image quality. In the second sig, I think the image quality was probably nice to begin with, but the colouring took it down a notch.
When you're colouring something, always take a step back and ask yourself "Did this colour/hue/contrast I just applied affect the image quality?"... if the answer is yes, you should click undo and try to achieve the same colouring in a different way.

If you leave it be, the image quality will only get gradually worse...trust me, been there done that.xD
I hope that was helpful to you!

Love
Val