If you've ever tried to scale imagery (sprites / backgrounds) up while having texture (/pixel) interpolation turned on, you might have encountered some drawing artifacts. These normally look like the left part of above image, being either random black/white dots or a whole semi-transparent outline around the image. This article goes a bit in-depth about why these occur and how to fix them.
With the glitch (left) and without (right)
Source of problem
The source of this unasked pixel issue is not actually even GameMaker' fault - it's in the pixels.
If you have been using GameMaker since version 7 or lower, you may remember those times when you had different colors in image, such as red, green, blue, and transparency:
Since earlier versions of GameMaker used 24-bit bitmaps, one of image colours could have been sacrificed to indicate transparency, and it might have looked like a separate colour as such. But, since the introduction of 32-bit images (and thus actual transparency) in GameMaker 8.0, you can no longer call transparency a colour.
If you [somehow] don't know how transparency works at all, here's a short explanation:
Every pixel of your images used in GameMaker is composed from 4 components - red, green, blue, and alpha. While first three define what colour you are actually going to get (see Wikipedia if you need more information on this), alpha channel defines how non-transparent (opaque) your colour is going to be. Alpha value of one means that colour is entirely opaque. Alpha value of zero means that colour is entirely transparent (invisible, you could say). And, given that each component can have 256 different values, amount of these absolutely invisible colours is quite large.
And, surprisingly, the "invisible" pixels are what actually ruins the scene here.
Transparent pixels cannot be directly seen, and it would have looked like it should be no problem to differentiate between transparent and non-transparent ones, but that's where the GPU (or CPU, depending on case) kicks in:
Now, I'm not implying that processing units are simple, or any of people creating programs for them are, but in many cases the interpolation algorithm does not always necessarily notice that one of target colours is transparent, and so you may get situations when interpolated "edge" pixels accidentally take transparent pixel colours into account, thus mixing the colours with technically invisible ones.
Finding "invisible" pixels may seem like a hard task, but it is not. As fact, you can even check the colour of these via colour picker tool:
(here I've made two versions of image, one with transparent contents and one with opaque ones)
Process of fixing colours of unseen pixels is also incredibly simple. As fact, you can even do it from GameMaker itself, thanks to the methods used by Glow filter:
Keep in mind that this will (obviously) only allow you to replace the surroundings to a single colour. If different parts of image have entirely different colours, it would be better to export image and edit it with a more appropriate image editing software (for example, Adobe Photoshop has an option to display/edit different channels of image, while editors like Paint.net and GIMP have different plugins to manipulate alpha channel of image).
You can also download example used to demonstrate the issue:
In GameMaker:Studio above approach may cease to work sometimes (due to texture page generation methods. If that occurs, add glow with alpha of 7 - that will add a 1% opaqueness glow but will also fix the issue.