Magmi

"value replacer"  magmi.

http://magento.stackexchange.com/questions/8893/how-do-i-use-magmi-column-mapper-for-attributes-with-the-same-value
As for value replacer, here is how it's done:

Replaced attributes: small_image,thumbnail

small_image: {{ '{item.small_image}' == '' ? '{item.image}' : '{item.small_image}'}}
thumbnail: {{ '{item.thumbnail}' == '' ? '{item.image}' : '{item.thumbnail}'}}

This will make small_image and thumbnail similar to "image" field unless explicitly assigned.

to delete the product using magmi

http://stackoverflow.com/questions/19333400/magmi-import-doesnt-remove-the-products
http://sourceforge.net/apps/mediawiki/magmi/index.php?title=Product_Deleter


how to import external url images using magmi
http://sourceforge.net/apps/mediawiki/magmi/index.php?title=Image_attributes_processor#Remote_image_support

csv format:

sku;store;attribute_set;type;categories;root_category;product_websites;brand;camera;color;cost;country_of_manufacture;created_at;custom_design;custom_design_from;custom_design_to;custom_layout_update;delivery_time;description;external_gallery;gallery;gender;gift_message_available;has_options;image;image_external_url;image_label;keywords;manufacturer;media_gallery;memory;meta_description;meta_keyword;meta_title;minimal_price;msrp;msrp_display_actual_price_type;msrp_enabled;name;news_from_date;news_to_date;options_container;page_layout;price;required_options;short_description;size;small_image;small_image_external_url;small_image_label;special_from_date;special_price;special_to_date;status;tax_class_id;thumbnail;thumbnail_external_url;thumbnail_label;updated_at;url_key;url_path;use_external_images;visibility;webkulamountper;webkulshipingtype;weight;qty;min_qty;use_config_min_qty;is_qty_decimal;backorders;use_config_backorders;min_sale_qty;use_config_min_sale_qty;max_sale_qty;use_config_max_sale_qty;is_in_stock;notify_stock_qty;use_config_notify_stock_qty;manage_stock;use_config_manage_stock;stock_status_changed_auto;use_config_qty_increments;qty_increments;use_config_enable_qty_inc;enable_qty_increments;is_decimal_divided;_links_related_sku;_links_related_position;_links_crosssell_sku;_links_crosssell_position;_links_upsell_sku;_links_upsell_position;_associated_sku;_associated_default_qty;_associated_position;_tier_price_website;_tier_price_customer_group;_tier_price_qty;_tier_price_price;_group_price_website;_group_price_customer_group;_group_price_price;_media_attribute_id;_media_image;_media_lable;_media_position;_media_is_disabled