That options will let you control extra actions to made on imported or pasted image after the placement.

Mirror horizontally

Will flip image horizontally after it is placed in placeholder.

Mirror vertically

Will flip image vertically after it is placed in placeholder.

Optional Rasterization

Will let you rasterize image to given resolution and color mode after it is placed in placeholder. Example use case when you should consider to use rasterization:

• You will have a lot of thumbnail like images in your project  that will be imported from high resolution images and you want to downsample images on the fly (to avoid  much higher resolution than target printer supports). You will reduce file size while not lose any print quality.
• You intentionally want to flatten on the fly any imported vector images. This may help you to get more predictable look of project when it will be printed, regardless how much complexity and effects could be in imported file that is not available for your view at the moment of making template with placeholder.

Rasterize thumbnails

Select this checkbox to enable rasterization.

Resolution

Specify downsample resolution. If you are going to print project later suggested value is 300 dpi. If your goal is to share an electronic publication via email, lower values may be used to produce less filesize, but you should not specify value lower than 72 dpi.

Color Mode

Specify color mode that must be used on rasterized image. If you are going to print project based on your template later - select CMYK, otherwise RGB is best option.