A little C program that recursively resizes images with variable minimum edge and quality values. Also supports an optional 'dry_run' switch. Utilizes MagickWand.

recursive_mogrify.c

#include <sys/types.h>
#include <stdlib.h>
#include <dirent.h>
#include <stdio.h>
#include <string.h>
#include <wand/MagickWand.h>

int count = 0;

void processdir(const char *name, const int min_edge, const int quality, const int dry_run) {
    DIR *dir;
    struct dirent *entry;
    const char *img_exts[] = {"jpg","jpeg","JPG","JPEG","png","PNG", "gif", "GIF"}, dot = '.';
    char path[1024], fullname[1024], command[1024];
	char *ext;
    int i, isimg, w, h;
    MagickBooleanType status;
	MagickWand *magick_wand;

    if (!(dir = opendir(name)))
        return;

    while ((entry = readdir(dir)) != NULL) {
        if (entry->d_type == DT_DIR) {
            if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
                continue;
            snprintf(path, sizeof(path), "%s/%s", name, entry->d_name);
            processdir(path, min_edge, quality, dry_run);
        } else {
        	isimg = 0;
        	ext = strrchr(entry->d_name, dot);
			
			if (ext == NULL)
				continue;
		
			for (i=0; i < 8; i++) {
				if (strcmp(img_exts[i],ext + 1) == 0) {
					isimg = 1;
				}
			}
			
			if (isimg) {
				snprintf(fullname, sizeof(fullname), "%s/%s", name, entry->d_name);

				MagickWandGenesis();
				magick_wand = NewMagickWand();
				status = MagickPingImage(magick_wand,fullname);
					
				if (status != MagickFalse) {
					w = MagickGetImageWidth(magick_wand);
					h = MagickGetImageHeight(magick_wand);
					if ( w > min_edge || h > min_edge ) {
						snprintf(command, sizeof(command), "mogrify -scale \'%dx%d>\' \"%s/%s\" -quality %d", min_edge, min_edge, name, entry->d_name, quality);
						if (dry_run) {
							printf("%s\n", fullname);
						} else {
							printf("%s\n", command);
							system(command);
						}
						count++;
					}
				} else {
					printf("There was a problem processing file: %s\n", fullname);
				}
				
				magick_wand = DestroyMagickWand(magick_wand);
				MagickWandTerminus();

			}
        }
    }
    closedir(dir);
}

int main(int argc, char *argv[]) {
	if (argc < 3) {
		printf("Required params (int)min_edge and (int)quality missing.\n");
		printf("e.g. #>recursive_mog 1024 80\n");
		printf("Where 1024 is the minimum width or height in pixels of the images you wish to resize.  And 80 is the quality output of the resulting image after resizing.\n");
		printf("Optionally, you can throw the 'dry_run' switch at the end of the command to only print which files would be modified.\n");		
		return 1;
	} else {
		const int min = atoi(argv[1]);
		const int qual = atoi(argv[2]);
		int dry;
		if (min == 0 || qual == 0) {
			printf("Values for min_edge and quality were provided but are not valid integers.\n");
			return 1;
		} else {
			if (argv[3]) {
				dry = 1;
			} else {
				dry = 0;
			}
			processdir(".", min, qual, dry);
			if (dry) {
				printf("All Done. %d Files Would Have Been Modified.\n", count);
			} else {
				printf("All Done. %d Files Modified.\n", count);
			}
			return 0;
		}
	}
}