How do I place one watermark tilted at 45 degrees right in the middle of the image?

45 or -45?

A positive value turns the mark clockwise, so the text runs downhill from left to right; a negative value tilts it uphill. For English, -45 gives the familiar rising diagonal from bottom left to top right. Arabic reads from the right, so for an Arabic mark it is +45 that reads as rising, from bottom right to top left. If your line mixes both languages, try both values and let the preview decide.

The tilted box is bigger than you think

When the text turns, the tool works out the box that contains it at that angle, and at 45 degrees that box becomes roughly square, with sides much longer than the text is tall. These figures come from the tool on a 1600 × 1200 photo, assuming a typical text height:

In the center position the tool does not push the mark inward: it keeps the center in the middle and lets anything outside be cut. The side positions behave differently: there the tilted box is kept inside the margin.

Steps

1. Open the Image watermark editor tool, choose your photos, then type the text or upload the logo.
2. Choose 'Single mark' and then the middle square of the 3×3 grid.
3. Move 'Rotation' to 45 or -45. The slider moves in whole degrees, and the arrow keys help with fine adjustment.
4. Set the size so the ends of the text stay inside the photo, and the opacity at 25% to 40% so it does not overpower the product.
5. Check both portrait and landscape photos in the batch, then download.

One in the middle, or a repeated pattern?

A single diagonal mark in the middle is harder to crop out than a corner mark, because it sits on the product itself, and it is easier on the eye than a repeated pattern. But it leaves large parts of the photo clean, and painting it out is easier than removing a pattern that covers everything. It is a good middle ground for catalogue and offer images; if the photo itself is what you sell, repeating is stronger.