Design for the thumbnail, not the canvas
Almost nobody sees your post at full size. They see it as a thumbnail in a feed while scrolling, which means one clear subject beats four small ones every time. Pick a single icon, make it large, and let it carry the post.
A 3D render does well here because the depth and the shadow give it an edge against a flat background, which is exactly what a thumbnail needs to be legible at 200 pixels wide on a phone somebody is holding at arm length.
Compression is not kind
Every platform recompresses what you upload, usually more than once. Fine detail and subtle gradients are the first things to go, so start from the highest resolution file you have rather than a screenshot, and export at the platform size rather than letting the platform resize for you.
Transparency matters here too. Put the icon on a solid brand colour in your design tool and export a flat image, rather than uploading a transparent PNG and hoping the platform picks a background you like. Some of them pick white, some pick black, and stories pick whichever ruins it.
Consistency across a grid
A profile grid is the one place a person sees nine of your posts at once, and it is where a mixed icon style is most obvious. Using one render style across a month of posts makes the grid look like a brand rather than a stream.
Every icon here is drawn in the same 3D style with the same lighting, so a run of posts built from them holds together without anybody having to enforce a rule about it.
Sizes worth keeping to hand
A square post at 1080 by 1080 covers most feeds and is the safest default. A story or a reel wants 1080 by 1920, with the important content kept away from the top and bottom two hundred pixels, because that is where the interface puts its own controls and your icon will end up behind a profile picture.
A link preview card is 1200 by 630, and it is the one people forget. It is what appears when somebody shares your page in a chat, which means it is often seen by more people than the page itself.
Build each size deliberately rather than cropping one image three ways. A square composition cropped to a story leaves an enormous empty middle, and a story cropped to a square loses whichever end had the point in it.
