Combining images sounds like a feature that should not need a tool, and in many cases it does not. What makes it worth doing properly is the situations where the obvious method falls down: screenshots that need to be one tall image for a support ticket, before-and-after shots that need to sit next to each other, or a set of captures that only make sense as a single frame.
What you can put together
The tool joins several images into one picture, in the order you add them, with a gap you control. That covers the two common arrangements: several images side by side for a comparison, or several stacked into a single tall capture that fits neatly into a message, an issue report or a document. Because the result is one file rather than several, it goes everywhere one file goes, which is often the entire point.
Setting the gap
The gap is a small detail that makes a visible difference. A few pixels looks tidy and lets the images read as a single composition. A larger gap makes the boundaries obvious, which is what you want when someone needs to tell where one screenshot ends and another begins, because it removes any doubt about whether you are looking at one long image or several separate ones.
Quality and dimensions
Nothing is re-compressed on the way through, so the merged file is as sharp as whatever you put in. What does change is the overall size, and this is worth thinking about before you merge. Two images side by side are twice as wide, so if each was already close to the maximum upload size of wherever you are posting, the combined file will exceed the limit that either one passed on its own.
Adding a gap increases the size again. If the result is too large to send, compressing it afterwards is usually better than hunting for smaller originals.
Order matters
Add the images in the order you want them to read. For a before-and-after this is obvious; for a sequence of screenshots, the natural reading order is almost always top to bottom. The tool combines them in the sequence you provide rather than sorting them, so taking a moment over the order avoids having to redo the whole thing.
Everything happens locally in the browser, which matters if you are combining screenshots that contain personal information, customer details or anything else you would rather not hand to an upload form.
When the merged file is too big
Combining two images produces a file twice as wide or twice as tall, and that is where the practical trouble begins. An upload limit that each image passed on its own will be exceeded by the pair, which is the most common reason a merge that worked technically still cannot be sent.
There are two ways round it. Compressing the result is usually quickest, and for screenshots of text it needs very little compression to save a great deal, because screen content is mostly flat areas of colour that compress efficiently. Resizing before merging is worth considering when you are combining many images, since the savings scale with the number you add.
Choosing the sources carefully prevents most of the problem. Merging screenshots taken at the same screen resolution produces a clean result. Mixing a phone screenshot with a desktop capture introduces mismatched scaling that stays visible in the finished file whatever you do afterwards.
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