image_func can take a named function from the magick package, but only functions that return an image that is compatible with the current template (e.g., doesn't change size or shape).
Examples
image_func_types()
#> [1] "annotate" "apply" "average" "background"
#> [5] "blur" "canny" "channel" "charcoal"
#> [9] "colorize" "combine" "composite" "contrast"
#> [13] "convert" "convolve" "despeckle" "edge"
#> [17] "emboss" "enhance" "equalize" "fill"
#> [21] "flatten" "fuzzycmeans" "fx" "fx_sequence"
#> [25] "implode" "lat" "level" "map"
#> [29] "median" "modulate" "morphology" "motion_blur"
#> [33] "negate" "noise" "normalize" "oilpaint"
#> [37] "ordered_dither" "page" "quantize" "reducenoise"
#> [41] "repage" "separate" "set_defines" "shade"
#> [45] "strip" "threshold" "transparent"