Since Gautham is on the subject of presentation pet peeves, I thought I'd bring up one of my own (along with some solutions).
We do a lot of imaging in the lab, and one of the best things about it is that it can produce compelling images that are fun to show off at a talk. Nothing like a few oohs and aahs to bring the audience back from the dead...
But one thing that always bugs me in talks is the dreaded "Well, you can't really see it so well here, but trust me, it looks really good on my screen..." (and yes, it's happened to me many times). This is often accompanied by a valiant but typically doomed attempt to show the image on the laptop to someone in the audience to try and corroborate the claim. What are the common reasons that images look bad on the screen? Here are a couple along with some suggestions.
1. Image features are too small to see. We have a lot of RNA FISH images that we like to show off in talks. Which of course brings up a problem: the RNA spots are really small, and often we want to show off multiple cells. Or, even worse, comparisons between fields of cells. The temptation is to put those fields next to each other and shrink down the image. The problem then is that the images become so compressed that nobody can see the spots anyway. So then what's the point? If you have an image scale issue, one solution is to break the problem down. Show the outermost scale to get people oriented (along with markers that are actually visible at that scale), then show a little box around the area, and then zoom in.
2. Merges. Gautham covered this nicely. Merges usually just look really bad. Try not to use them. There are a few situations in which they can be useful, but they are rare.
3. Contrast is to low. This is actually an issue that's even more pronounced in print. The problem is that stuff that's obvious when you're sitting right next to your screen is hard to see on the projector. I think this is because when you are close up and have time to pay attention, it's easier to focus on what are actually rather subtle features in the image.
4. Settings on Apple Laptops
Here's another seemingly arcane but often crucial little trick. Despite my best efforts to adhere to the above rules, a few years ago I noticed that after I updated my laptop's OS, I found myself repeatedly saying "Well, somehow this isn't looking so good on the screen, but what you should see is..." Very embarrassing! And I just could not for the life of me figure out why my images were looking so crappy on the screen. Then, after some digging, I figured it out. Turns out that when you plug in a external projector (or monitor or whatever) to your Mac, it chooses a color profile for that device. This governs how the colors look on the screen. The issue is that a couple years back, Apple updated things so that the default color profile when you connect has terrible contrast for many images. The solution is to open "Displays" in System Preferences, then go over to the output screen, click on the "Color" tab, then uncheck the box marked "Show profiles for this display only", and then select "sRGB IEC61966-2.1". Anyway, after I did that, all those weird problems went away. But only after dealing with the first 3 items...