An image CDN does three jobs: store your originals, transform them into the right size and format on demand, and deliver the result fast from somewhere near each visitor. Most providers do all three — the differences that matter are in the details and the bill. Here's what to actually look at.
1. Egress fees (the silent budget-killer)
Bandwidth is where image bills explode. Modern object storage offers zero egress fees, which is structurally cheaper than CDNs that meter every gigabyte served. If a provider charges per-GB delivered, model your real traffic before committing — a viral post can turn a $9 month into a $900 one.
2. On-the-fly transforms
You want to upload one source image and request any size or format from a URL — ?w=640&format=webp — generated the first time it's asked for and cached forever after. This kills the need to pre-generate dozens of sizes, and it means a new device or layout never requires a re-upload.
3. Modern formats, automatically
WebP and AVIF are dramatically smaller than JPEG and PNG at the same quality. The CDN should produce them for you. Animated GIFs in particular should convert to animated WebP — often a 5–10× size drop for the same loop. swiftimg does this conversion automatically on upload.
4. Signed URLs
If transforms are open to anyone, attackers can run up your bill by requesting thousands of permutations. Signed URLs make each transform tamper-proof and optionally time-limited, so only links you generate are honored.
5. A pricing model you can predict
Volume-based pricing is great until it isn't. Flat plans with clear, usage-based overage beyond the plan limits are far easier to budget around. Watch for per-transformation credits that quietly cap how much you can actually do.
6. Delivery footprint
The best transform pipeline is wasted if the bytes travel across an ocean to reach your user. Global delivery — caching the result close to each visitor — is what turns “generated in 150 ms once” into “served in 1–2 ms forever after.”
A quick checklist
- No (or negligible) egress fees
- On-the-fly resize & format from a URL
- Automatic WebP/AVIF, including animated GIF → WebP
- Signed URLs to protect transforms
- Flat, predictable pricing with sane overage
- Global CDN caching with immutable URLs
swiftimg is built around exactly this checklist — on a global CDN, at a flat price. If you're comparing options, see how it stacks up against Cloudinary, imgix, and ImageKit.