Covers and images

Two kinds of artwork go into a book: the covers (front and back), and the images inside the text. unipress handles both — this page covers how to add them and how to make them the right size and resolution for print.

Covers

Your book has a front cover and, optionally, a back cover. Put the image files in your project's assets/ folder and point to them under book.covers:

# document-book.yml  (your final variant)
book:
  covers:
    front: assets/front.png
    back:  assets/back.png

That's it. On the next build the front cover becomes the first page of the PDF and the thumbnail of your EPUB; the back cover becomes the last page of the PDF. (EPUBs have no back cover — ebook readers only use the front.)

Covers must live in the config unipress actually reads — your default document.yml or the file you pass to --variant. Covers set only in a unipress.config.js (--config) won't be picked up. If a cover silently doesn't appear, see troubleshooting.

Size and resolution

Make the cover image the same proportions as your trim so it fills the page without stretching or letterboxing. A 6 × 9 book is a 2 : 3 (0.667) ratio, so a portrait image around 1800 × 2700 px covers it at print quality (300 DPI at 6 × 9). For the ebook thumbnail, a tall image of at least 1600 × 2560 px is a safe, widely-accepted size.

Your trim	Cover proportions	~300 DPI pixel size
6 × 9 in	2 : 3	1800 × 2700
7 × 10 in	7 : 10	2100 × 3000
A5 (148 × 210 mm)	~1 : 1.41	1748 × 2480

Use PNG or high-quality JPEG. The title and author usually live in the artwork itself — if yours does, turn off the typeset title page so it isn't repeated: book.structure.titlePage: false (see Size and layout).

The print cover is a different file

This is the one thing that surprises first-time authors. The book.covers images above are your digital covers — used for the EPUB and the standalone PDF. A print book needs something else: a single wrap-around cover image — back + spine + front in one piece, with bleed past the edges.

The catch is the spine width, which depends on your page count and paper stock, so you can only make the wrap cover once the interior is final. Every print-on-demand service gives you a cover template generator that outputs the exact dimensions for your book:

Feed it your trim, page count, and paper, drop your art onto the template, and upload that as the cover — separate from the interior PDF. See Publishing in print for the full upload flow.

Getting cover art made

A good cover sells the book; it's worth doing well. Realistic options:

Hire a designer — Reedsy, 99designs, or Fiverr span a wide range of budgets.
Use a cover tool — Canva and Adobe Express have book-cover templates.
Make your own if you have the skill; start from your trim's proportions above.

Whichever you choose, match the size and bleed your print service asks for.

Images in your book

Put image files in assets/ and reference them from your markdown with a normal image link. The book foundation numbers figures and renders your caption:

![A cross-section of the cell, labelled.](/assets/cell-diagram.png)

The text in the brackets becomes the caption (and the accessibility description — worth writing well). You don't manage figure numbers; the foundation does.

Resolution: enough pixels to print

Screens are forgiving; print is not. A print needs about 300 pixels per inch at the size the image appears on the page. So an image printed 4.5 in wide (roughly the text width of a 6 × 9 book) wants to be about 1350 px wide minimum; a full-page plate wants more. A photo that looks crisp on screen at 600 px will print soft and blocky.

Rule of thumb: printed width in inches × 300 = minimum pixels wide. Supply the highest-resolution originals you have; unipress embeds them at full quality, so the source image is what determines how it prints.

Color

Screens are RGB; presses are CMYK. Print-on-demand services accept RGB images and convert them for you, which is fine for most books. If exact color fidelity matters (art books, brand colors), ask your print service whether they want CMYK and proof a physical copy before publishing — on-screen color never fully matches ink.

Formats and file size

PNG for diagrams, line art, and anything with text or sharp edges; JPEG for photographs. There's no need to hand-optimize — but very large originals make for large PDFs, so a few-MB photo is plenty; you rarely need 20 MB camera raws.

Set your size and layout.
Produce your ebook or go to print.