Every Slack image size
Slack needs only three images, and two of them are the same square.
Profile photo
512 × 512 px, 1:1 Your photo next to every message.
Workspace icon
512 × 512 px, 1:1 Shown in the workspace switcher.
Custom emoji
128 × 128 px, 1:1 Must be under 128 KB. Exported as PNG.
Making a custom emoji Slack accepts
Custom emoji are where most Slack uploads fail, and the cause is usually the file size rather than the dimensions. A 128 × 128 PNG of a simple icon is a few kilobytes; the same size cut from a detailed photo can be far larger.
The tool shows the file size of every image it makes. If the emoji is 128 KB or more, it says so, with the size, before you download it.
Emoji are shown at text size, against both light and dark themes. A transparent background keeps them from looking like small boxes in the middle of a sentence: start from a PNG with transparency and the tool keeps it.
Profile photo and workspace icon
Both are 512 × 512 px. The profile photo appears next to every message you send, at a small size, so a tight crop of your face works best. Keep it consistent with your photo on other work tools so people recognize you.
The workspace icon is seen by everyone who switches between several workspaces. A logo on a plain background, or one or two letters, is easier to pick out than a photo.