A sideways image needs rotation, not a different width entered into a resizer. Rotation changes where the pixels sit. For angles other than a right angle, it can also require a larger surrounding canvas.
Image Rotator uses clockwise angles. Positive 90 turns the image clockwise, and a negative angle turns it in the opposite direction.
A right angle swaps the sides
A 400 by 300 source rotated by 90 degrees becomes 300 by 400. The full rectangular image fits that swapped canvas without the triangular corners produced by a smaller angle.
At 45 degrees, the bounding rectangle expands to contain the rotated image. The selected background fills the space around it. That new canvas can be larger than you expected, so inspect the output dimensions before sending it to an upload form.
Supported JPEG camera orientation is applied before this operation when PHP EXIF support is available. If the preview already appears upright, an additional 90 degree turn may be unnecessary.
Rotation is not automatic straightening
The tool does not detect a horizon or calculate the angle of a photographed document. Choose an angle from the source and review the result visually.
Small repeated rotations can soften edges because each downloaded copy may be resampled again. Return to the original when trying a different angle instead of repeatedly turning the previous result.
A larger rotated canvas must stay within the tool's output pixel limits. An input that fits can still produce a rejected result if its expanded bounding box is too large.
For a photographed form, check that every corner remains visible. The picture may be upright while its text is still skewed by perspective, which this rotation tool does not correct.