how to resize multiple images at once?

Because you've already got them all bunched up into one div with no other type of imagery in there, you can just blanket-tag every <img> instance in there with your width/height of choice by assigning that div an ID, and then doing #id_name img { width: 99px; height: 56px; }. (In addition, you can—and should—shove that div's in-line styling into that ID while you're at it.)

Here's an example of what that ends up looking like:

<style>
#stamps {
    height: 300px;
    overflow-y: scroll;
    background-color: rgba(216, 229, 235, 0.70);
    border: 3px dotted #fff; border-radius: 15px;
}

#stamps img {
    width: 99px; height: 56px;
}
</style>

(...)

<div id="stamps">
[[ your stamps go here, duh ]]
</div>

thank you so much! your explanation was really easy to understand, and i'm grateful you decided to help :)

Happy to lend a hand! Going forward, I'd strongly suggest utilizing both Stack Overflow for queries that seem like they've been a thorn in someone else's side at one time or another, and/or the MDN Web Docs for in-depth rundowns + examples (usually) for almost every aspect of HTML and CSS.
Best of luck with your future coding endeavours. ^^