Error: Sizes of input arguments do not match, why?

edit

you can only add Mat's of same size and same type.

since you're loading bgr images, your accum buffer needs to have 3 channels, too, also you would need to convert your loaded images to double, too (using cv::accumulate would spare you the manual conversion).

std::vector<cv::Mat> depths;
for (int i = 0; i<102; ++i) {
    cv::Mat im = cv::imread(img_path + std::to_string(i) + img_suffix);
    CV_Assert(! im.empty()); // please *always* check resources
    depths.push_back(im);
}
cv::Mat mean_depth = cv::Mat::zeros(depths.at(0).size(), CV_64FC3); //!!

for (std::vector<int>::size_type i = 0; i != depths.size(); i++) {
    cv::accumulate(depths[i], mean_depth);
}

// i can only guess, but you probably want this in the end:
mean_depth.convertTo(mean_depth, CV_8U, 1.0/depths.size());