In Google Chrome

• Go to Instagram.com and sign into your account;
• Right-click anywhere on the page and select Inspect from the context-menu or click Ctrl-Shift-I;
• The Inspector Tool window appears on the right or at the bottom of the page;
• In the top-left corner of the Inspector Tool window, click on the Tablet/Phone icon;
• The page will show a mobile device view;
• Click on the plus (+) icon at the bottom of that window;
• From the directory window that shows, click on the image file to post and click open;
• To reduce the image, click on the circled double-arrow in the bottom-left corner of the picture;
• Click Next above the top-right corner of the picture;
• In the "Write a caption" box, write your comments;
• Click Share;
• Repeat ALL the previous steps for each picture or video you want to post;
• Close the Inspector Tool window and refresh the page to view your post(s).

Et voilà! Have fun!